Thermal cracking of petroleum-related materials produces a variety of organic chemical components, including mercaptans. One of the products of the thermal cracking process is a gaseous stream, typically referred to as a “charge gas stream”, containing various C1 to C6 hydrocarbons, hydrogen and CO2. The hydrocarbon portion of the charge gas stream contains a mixture of olefins, diolefins and acetylenic components. The charge gas stream also typically contains various sulfur containing byproducts of the cracking process, including H2S and various mercaptans, such as methyl, ethyl, propyl and butyl mercaptans.
The charge gas stream resulting from thermal cracking may contain concentrations of up to 1000 ppm mercaptans. Mercaptans are known to poison or deactivate noble metal selective hydrogenation catalysts. The presence of mercaptans can reduce the effectiveness and life of catalytic hydrogenation reactors, essential parts of conventional charge gas purification and separation processes. Existing caustic scrubber technology of charge gas streams, effective for removal of H2S and CO2, is ineffective for the removal of mercaptans.
Most industrial processes can tolerate mercaptan concentrations of up to 1000 ppm. Olefin selective hydrogenation systems are designated as either “front-end” (upstream of hydrogen removal) or “back-end” (downstream of hydrogen removal). In conventional back-end or front-end ethylene plants, the mercaptans are separated from the charge gas by distillation before they reach the hydrogenation stage. In back-end systems, due to the boiling point of mercaptans, the mercaptans are contained within the C5+ stream and therefore end up in the pygas stabilization section away from the hydrogenation catalyst. In the first stage of the pygas purification section, liquid phase hydrogenation is performed using a high Pd (<0.4%) or a Ni-based sulphur tolerant catalyst. In the second stage of the pygas purification process, gas phase hydrodesulphurization (HDS) is performed using Co—Mo or Ni—Mo catalysts specifically designed to reduce total sulphur to very low levels.
In one type of front-end system, where the overhead of a depropanizer is fed into a front-end hydrogenation reactor, only methyl-mercaptan is light enough to have an effect. Because the concentration of methyl-mercaptan is normally in the low ppm level at that point, it is expected to have some effect, but this effect can be minimized by the use of higher catalyst loading or higher operating temperatures. The rest of the mercaptans and other sulphur compounds are eliminated in the pygas stabilization section as previously described.
Some industrial processes cannot tolerate mercaptan concentrations of up to 1000 ppm. These processes, such as the front-end CD-Hydro and olefin metathesis reactions, require very low levels of mercaptans in the feed. In the front-end CD-Hydro process, hydrogenation takes place together with distillation before the heavier mercaptans have been removed. The CD-Hydro catalyst, which is a noble metal catalyst, can therefore be deactivated unless mercaptans have been removed from the feed. As a result, it is preferred that mercaptan levels in the feed are at concentrations less than 5 ppm. A traditional solution used commercially to remove oxygenates and sulphur compounds from hydrocarbon streams is an absorber guard bed made of a zeolite material. This is not a viable alternative for these processes because the high reactivity of the charge gas feed will foul the bed, rapidly making it ineffective.
The olefin metathesis process also has very stringent requirements for the mercaptan levels in the feed. Based on their boiling point, certain mercaptans, such as methyl and ethyl-mercaptan, are contained in the C4 olefin stream that feeds the metathesis reactor. Currently, an adsorber bed with a zeolite molecular sieve material is used to remove oxygenates and sulphur compounds from this stream. However, mercaptans can potentially lower the effectiveness of these guard beds to oxygenate removal. Removing the mercaptan compounds upstream from this process in the charge gas treatment area will reduce the absorbent volume requirement and increase the effectiveness of this guard bed in oxygenate removal.
The extraction of mercaptans from hydrocarbon streams is widely practiced in refining. One commercially known process, MEROX®, is described in U.S. Pat. Nos. 2,988,500, 4,626,341 and 5,424,051, each of which is hereby incorporated by reference in its entirety. MEROX®, as well as the related Thiolex process, uses caustic regeneration and has been used in fuel gases, cracked gasoline, LPG streams and heavier fractions. These streams are mostly liquid and have a relatively low olefinic and diolefinic content (i.e., they are not very reactive). Charge gas, on the other hand, has a very high olefinic content with a significant amount of diolefins and acetylenics making these scrubbing methods ineffective.
Thioetherification has been used for the removal of mercaptans from refinery streams. For instance, U.S. Pat. No. 6,231,752 (the entirety of which is hereby incorporated by reference), describes the removal of mercaptans from a light naphtha stream as part of a Catalytic Distillation Hydrosulfurization process using a Ni-based catalyst. This process takes advantage of the thioetherification reaction for sulphur removal via formation of heavy sulphur species components and their removal through the bottoms of the catalytic distillation column. However, the liquid phase reaction mechanism for this technology is only effective for diolefins. In particular, butadiene and isoprene can react with mercaptans to produce thioethers such as butyl-ethyl or C5-ethyl sulfide. U.S. Pat. Nos. 6,849,773 and 6,919,016 (the entirety of each of which is hereby incorporated by reference) describe the same process for a C4 stream.
U.S. Pat. No. 5,851,383 (the entirety of which is hereby incorporated by reference) describes a diolefin hydrogenation-thioetherification process over a Ni-based catalyst on a C3-C5 FCC stream. A combination of a fixed bed hydrogenation reactor and a distillation column are disclosed for the removal of heavy sulphur components. This system is a back end system in which hydrogen is fed as a separate stream, at a low level, with the C3-C5 stream in a fixed bed reactor. The feed stream used in the system described in U.S. Pat. No. 5,851,383 is much lower in reactive species than charge gas.
As such, there exists an ongoing and unmet need in the industry for processes to remove mercaptans from charge gas streams.